Soldering and brazing are commonly used joining methods which involve interposing a suitable fusible material between two parts to be joined, heating to cause the fusible material to melt and wet both parts, and then cooling to form a bond between the parts. In common parlance such processes that are carried out at temperatures up to about 450.degree. C. are generally characterized as soldering, above about 550.degree. C. as brazing, and between about 450.degree. C. and 550.degree. C. as hard soldering or soft brazing. Hard soldering and brazing are more commonly employed for end use applications calling for higher strength and temperature resistance than realized in solder joints. For purposes of the present invention, which involves processes involving elements of both soldering and brazing, these processes will hereinafter be referred to collectively as "brazing."
In any event copper and copper alloys can be bonded to various substrates by soldering or brazing depending on the end use of the joint. In order for joining to occur, the molten solder or braze alloy must wet the surfaces of the members to be joined, as indicated above, and the melting temperature of the solder or braze alloy has to be below the melting temperature of the members being joined. In consequence of these requirements various solder alloys have been found useful, of which mention may be made of lead-tin, tin, and tin-silver, for example. Among commonly used braze alloys, mention may be made of copper-silver eutectic alloys, copper-gold, titanium-copper-silver, and nickel-gold systems, for example. (For the purposes hereof the terms "solder alloy," "braze alloy," or "brazing alloy" are intended to include such materials in elemental form, as well as in the form of true alloys.) Such braze alloys enjoy widespread use due to their excellent wetting of common engineering metal surfaces and melting temperatures in an acceptable range of about 780.degree. C. to about 1100.degree. C.
Another important consideration in both solder-bonding and braze-bonding is that the diffusion and alloying between between the molten solder or braze alloy and the members being joined should be kept to a minimum in order to avoid the undesirable formation of voids, microcracks, and brittle intermetallic compounds, resulting in joints that are weak and exhibit poor resistance to thermal shock. Therefore bonding is usually carried out by heating the assembly to just above the melting point of the solder or braze alloy and cooling immediately after adequate flow and wetting has been assured. For example two copper (melting point 1082.degree. C.) members can be joined using tin (melting point 232.degree. C.), which at 250.degree. C. would be molten, wet the copper members, and create a solder joint. If the thus-formed joint is heated further, near 400.degree. C., interdiffusion of tin and copper causes alloying to take place, resulting in the formation of an alloy that melts at over 600.degree. C.; so the parts remain bonded. If the temperature is then raised further, to 700.degree. C. or 800.degree. C. or even higher, increased alloying between the tin and the copper takes place, resulting in a new bond zone alloy with a melting point over 980.degree. C. However a joint fabricated in this manner is generally not acceptable for engineering applications such as heat exchangers for service at temperatures of 700.degree. C. to 900.degree. C., because the copper-tin interaction results in a poor joint with excessive voids and poor resistance to thermal cycling. This condition is not relieved by further heating, even at higher temperatures, but rather is worsened.
This undesired, uncontrolled interdiffusion and alloying among the solder or braze alloy and the members to be joined is a problem affecting many other systems requiring strong joints resistant to temperatures of up to 900.degree. C. or even more. As a further example mention may be made of beryllium, which is desired to be joined to copper in certain fusion reactor applications. Beryllium is a light-weight, high-stiffness metal which is difficult to join to copper and copper alloys with good joint reliability. Initially it is desired to avoid using typical brazes containing a noble metal such as silver or gold, as mentioned previously, because these become highly radioactive when exposed to a high energy neutron flux. Aluminum (melting point 660.degree. C.) and other aluminum-containing brazes might be considered, but when used to join beryllium to copper, the copper is transported across the molten braze alloy at temperatures as low as 550.degree. C. to create a brittle beryllium-copper intermetallic phase in the braze/beryllium interface zone, rendering the joint prone to fracture and failure.
A further deficiency in braze joining copper and copper alloys to ceramic materials such as used in high temperature vacuum feedthroughs is that the aforementioned conventional braze alloys that melt in the 780.degree. C. to 1100.degree. C. range are not well-suited for use with ceramics, since the latter are typically damaged as a result of thermal shock in processing.